Conservation of Energy/Enthalpy/Bond Enthalpy

Question 1

Negative/Positive ∆H

Answer 1

A negative ∆H means that the reaction is exothermic. A positive ∆H means that the reaction is endothermic.

Question 2

Where does ∆H come from?

Answer 2

∆H comes from from:

Figuring out how much energy is absorbed to break the reactant bonds (the change in energy from the reactants line to the top of the curve - labelled “Activation energy” in these diagrams).

Figuring out how much energy is released when all of the product bonds are formed (the change in energy from the top of the curve to the products line).

Comparing these two numbers (if more energy is absorbed than is released, then the reaction is endothermic; if more energy is released than is absorbed, then the reaction is exothermic).

Question 3

What is bond enthalpy?

Answer 3

Bond enthalpy is the energy required to break one mol of covalent bonds. Measure of the strength of a covalent bond (the stronger the bond the more tightly the atoms are joined together and the higher the bond enthalpy [the more energy is required to break it]). Also the amount of energy released when one mol of said bonds form.

Bond enthalpies are a useful way of calculating the approximate (within about 10%) change in enthalpy for a reaction. The values of bond enthalpies are AVERAGES because of the way that they are measured.

Question 4

How can we calculate ∆H from bond enthalpies?

Answer 4

We subtract the sum of the energy released when product bonds are formed from that required/absorbed to break all reactant bonds.

Note: If bonds broken are weaker (require less energy) than bonds formed, then the overall reaction is exothermic.

Also, when using bond enthalpy data, the first element would be in the rows.

Question 5

What are energy level diagrams?

Answer 5

Diagrams that model energy changes during reactions (show the relative energy of the reactants and the products).

Show the differences in potential energy contained in the reactants and products. Since the products are at a lower PE, the difference is converted to heat energy: lower energy means more stable!

For exothermic reactions…
Have negative ∆H values because chemicals release energy to the surroundings. Bonds in products are stronger than bonds in reactants.

For endothermic reactions…
Have a positive ∆H because chemicals gain energy from surroundings. Since PE of products is higher than the PE of reactants, some heat energy was absorbed. Bonds in reactants are stronger than bonds in products: reactants are more stable than products.

Question 6

What is enthalpy?

Answer 6

Heat content of a chemical or physical system. Measure of chemical energy stored in bonds - in both reactants and products.

“Absolute” values for enthalpies are unknown so the change in enthalpy (∆H) for a reaction is calculated.

∆H is the amount of heat energy absorbed or released (in kJ mo^l-1) during a change (physical or chemical).

Question 7

What is energy?

Answer 7

Measure of the ability to do work or produce heat. Can be in either of two states (potential or kinetic).

Question 8

Exothermic reactions

Answer 8

Release heat energy to the surroundings. Energy absorbed to break reactant bonds < energy released when product bonds are formed.

∆H is negative. Temperature of the surroundings increases.

Ex. Combustion (burning), hand warmers, rusting , reactions of acids and bases, condensing a vapor, freezing a liquid, forming bonds.

Question 9

Endothermic reactions

Answer 9

Absorb heat energy from the surroundings. Energy absorbed to break reactant bonds > energy released when product bonds are formed.

∆H is positive. Temperature of the surroundings decreases.

Ex. Ice packs - dissolves a salt in water, melting ice, boiling water.

Question 10

Mole calculation review

Answer 10

A mole represents a collection of 6.023 x 10^23 ( Avogadro’s number) chemical units.

Molar mass is the mass (in grams) of one mole of a substance. To calculate the molar mass of a molecule:

For each element in the molecule, use the periodic table to find the relative atomic mass (it’s the larger number which is usually written under the chemical symbol)

Multiply the relative atomic mass for each element by the number of atoms of that element and add them together. This tells you the mass of one mole of that substance.

If you are given the mass of reactant and need to calculate the number of moles of that reactant, use the following equation:

n = m / M
(where n = mols, m = mass, and M = molar mass)

Question 11

What is potential energy?

Answer 11

Stored energy. Due to position or due to the forces of attraction and repulsion between objects. Any object that has stored energy has a potential to do work.

Different types: gravitational (potential energy that is dependent on height), elastic (energy that is stored due to being stretched or compressed), and chemical (energy stored in the bonds of chemical compounds and molecules. On a molecular level, chemical potential energy comes from energy stored in covalent bonds, electrostatic forces, and nuclear forces).

Question 12

What is kinetic energy?

Answer 12

Energy of motion. The faster an object moves, the more kinetic energy it has. The greater the mass of a moving object, the more kinetic energy it has. Kinetic energy depends on both mass and velocity.
KE = ½ mv^2 m = mass (kg) and v = velocity (m/s)
Velocity has greater effect because if you doubled both it and mass, velocity would be greater (in the equation).

Question 13

What is temperature? What is heat?

Answer 13

Temperature is a measure of the average kinetic energy of the particles in a substance. Independent of how much substance is present.

Heat is the measure of the total energy in a given amount of substance. Depends on how much of the substance is present.

Question 14

Law of Conservation of Energy

Answer 14

Energy can be neither created nor destroyed by ordinary means. It can only be converted from one form to another (changes in the form of energy are called energy conversions). If energy seems to disappear, then scientists look for it – leading to many important discoveries.

Chemical potential energy is stored in chemical bonds.
When these bonds are broken, the stored potential energy is released and gives off varying degrees of kinetic energy, depending on the strength of the bonds.
We would see the kinetic energy given off in the form of heat (thermal energy is a form of kinetic energy because thermal energy comes from the movement of the particles, and kinetic energy is the energy of movement).

Conversely, you can put energy into a chemical system and it can be “absorbed” and stored in the creation of chemical bonds.

Question 15

What is a system? What are surroundings?

Answer 15

The system is the specific portion of matter in a given space that is being studied during an experiment or an observation (things we can’t see/watch/feel: what’s reacting [the reactants]). The surroundings are everything in the universe that is not part of the system (what we measure to see what’s happening in the experiment). If the system loses a certain amount of energy, that same amount of energy is gained by the surroundings. If the system gains a certain amount of energy, that energy is supplied by the surroundings.

Question 16

Transfer of energy

Answer 16

Energy is stored in chemical bonds and can be transferred to kinetic energy (heat).

Energy can also be transferred from warmer substances to colder substances through conduction (through collisions with neighboring atoms or molecules).

Question 17

Energy calculations

Answer 17

To calculate the heat gained (or lost) by a system, we use the following equation:

q = (m)(c )(ΔT)

Where: 	
q = energy change (J) 
m = mass of substance heated (g)
c = specific heat capacity (water = 4.18 J g-1 oC^-1)
 ΔT = change in temperature (oC)

The sign of q for an endothermic process is positive because the system is gaining heat. A chemical reaction or physical change is exothermic if heat is released by the system into the surroundings. Because the surroundings are gaining heat from the system, the temperature of the surroundings increases. The sign of q for an exothermic process is negative because the system is losing heat.

Remember: Energy transfer is based on the concept of CONSERVATION OF ENERGY!

This means that the amount of heat that is lost from one compound/solution/system is EQUAL TO the amount of heat gained by the other compound/solution/system.

We would represent this as:
q1 = -q2

If you’re asked to calculate the theoretical final temperature:

q1 = -q2

(m) (c )(ΔT) = -(m)(c )(ΔT)
* Remember: the final temperature is the same for both, and you’ll know the initial temperatures.

Question 18

Specific heat capacity

Answer 18

Amount of energy needed to raise 1 g of a substance by 1°C. Depends on the substance. Materials with a high heat capacity can “soak up” a given amount of energy with a smaller change in temperature.

Question 19

Things to remember

Answer 19

If there’s something aqueous, it’s always our surroundings (we’re measuring its temperature).

At the maximum/minimum temperature, we’ve used up one of our reactants.

When calculating the final temp, if what you get is hotter than your hot/colder than your cold, you know something’s wrong. If you have more hot water, it should be closer to hot (same with cold).

When all your water is liquid, all the energy goes into increasing the temp. When there’s ice, some goes into breaking them apart, and the rest goes into increasing the temp (not as high as a temp rise). With ice, for example, the temp goes up when the molecules start to move (so the temp stays the same as you melt it).